People live surrounded by numerous noise sources. Examples of noise include high-level outdoor noise such as exhaust noise of vehicles or friction sounds of tires, aircraft engine noise and noises of machines in factories and indoor noise. However, it is impossible to abandon modern conveniences in order to remove noise. Accordingly, various efforts to reduce noise generated in living environments have been made.
To control noise, a method of searching for the cause of noise and then actually decreasing the noise can be adopted. The active noise control technique is a typical noise reduction method. The active noise control technique superposes a sound wave having the same magnitude as noise and a phase (−90°) opposite the noise on the noise such that the noise and sound wave cancel each other out, thereby generating a sound shadow area. That is, the active noise control technique uses superposition and destructive interference, which are physical properties of waves. The most significant feature of this technique is to selectively cancel noise of a specific frequency. However, there are many limitations on active noise control techniques since it is difficult to generate a sound wave having an opposite phase when a noise wave has multiple forms. Particularly, a problem of delay taken to detect and analyze noise and to generate a sound wave having an opposite phase has not been adequately solved. Consequently, it is difficult to completely cancel noise. To compensate for such problems, sound quality enhancement technology has been developed. This technology changes the sound wave of noise into a sound that people can hear using the principle that a sound is heard differently when the waveform of the sound, not the magnitude thereof, is changed.
Vehicle indoor noise cancellation is a typical example. While costs can be large when attempting to remove the entire noise, costs can be reduced if only a specific noise to which people sensitively respond is reduced or eliminated. A known vehicle audio speaker is a typical case to which the sound quality enhancement technology is applied to effectively block a noise of the vehicle and outdoor noise. However, it is not easy to secure noise cancellation quality since there are various causes of vehicle indoor noise and systems for solving the same become complicated.